This invention relates to the high speed transmission of digital data in analog form with a low rate of error due to transmission impairments such as electrical noise. More particularly the invention provides new signal structures and decoding procedures for coded data communications characterized by relatively high immunity to error due to multiplicative noise, as well as additive noise and phase jitter.
Another signal structure resistant to error by multiplicative noise is described in co-pending application for patent Ser. No. 554,261 entitled "Signal Structure for Data Communication".
Digital data conventionally is transmitted, as on telephone lines, by sending a carrier signal with analog modulation selected to convey, in every time interval, a set of binary digits. A conventional practice sends M bits once per transmission interval, i.e., the baud interval, where M is an integer. The M bits have 2.sup.M possible values. Hence a coded analog signal in this practice has a structure which identifies one of the 2.sup.M possible values once per baud interval. For example, to send 2,400 bits per second (bps) over a line using a 2400 baud rate, in each baud interval a single bit is transmitted by selecting the analog signal to send one of two values, i.e., M=1 and 2.sup.M =2. Examples of conventional higher speed transmission rates are 9,600 bps wherein four bits are transmitted in each interval by sending one of sixteen analog values; and 14,400 bps, wherein six bits are transmitted in each interval by sending one of sixty-four analog values. Conventional baud rates include 1200; 1600; and 2400 symbols per second.
One conventional analog signal used for data transmission employs double sideband-quadrature carrier modulation (DSB-QC). This modulation technique involves two carrier signals 90.degree. apart in phase, that is having a quadrature relation, and each having double sideband modulation.
The coding of the modulation, in particular the selective amplitude and phase of the modulation, is defined by two analog parameters designated a.sub.i and b.sub.i where (i) is an integer. The values of these modulation parameters identify, in each baud interval, the one analog value being transmitted. Thus, in a 9,600 bps transmission system, there are sixteen possible sets of the two analog parameters to identify one of the sixteen possible analog values being sent. Similarly in a 14,400 bps transmission system, there are sixty-four possible sets of the two analog parameters.
The values of the modulation parameters are conventionally plotted or mapped as points in a two-coordinate complex plane. The set of points for a transmission system is termed a "signal structure", or "signal space," or "signal constellation".
The occurrence of electrical noise and other transmission impairments makes it difficult, however, to determine the value of the modulation parameters at the receiver end of a data transmission system. The probability of error due to this difficulty in value determination increases with the speed of data transmission. This is because higher transmission speeds involve a greater number of possible values in each baud interval, and it thus becomes increasingly difficult to distinguish between them in the presence of electrical impairments.
It is known to select a signal structure to reduce error rates and otherwise to enhance data transmission. U.S. Pat. Nos. 3,887,768 and 4,271,527 disclose two designs for signal structures. These and other known signal structures are nevertheless subject to significant error rates in the presence of certain forms of transmission impairments.
In order to further reduce transmission error, and at the same time increase and enhance transmission speed, various digital signal encoding methods have been employed. Among them are the Trellis codes which are one form of the class of convolutional codes. In accordance with these digital encoding methods, a block of input bits, for example six bits, are encoded into, for example, seven bits. Thus instead of a signal constellation having sixty-four points corresponding to six bits, the signal constellation has one hundred and twenty-eight points corresponding to seven bits. While at first glance it would appear that a signal constellation having more points would require more energy or a slower transmission rate, by proper use and selection of the encoding and decoding methods, an increased bit rate at constant energy and probability of error can be achieved. Thus, the CCITT has endorsed and recommended a particular Trellis coding method which provides superior performance in the presence of additive noise. That method has been commercialized in equipment operating at 14.4 kb/s and 16.8 kb/s.
The advantage of such encoding mechanisms lies in the interrelationship of successively received signals. Thus, when operating with a Trellis code, for example, one examines not only a just received point (A.sub.i, B.sub.i), but the relationship of that received point with, for example, seventeen previously received points. The effect is to provide an improved error rate even though the number of points in the signal constellation increases and the average energy of the signal remains constant.
An object of this invention is accordingly a data transmission method and apparatus for communicating data at higher data rates with an improved low error rate in the presence of different forms of transmission impairments.
Another object of the invention is a signal structure for high speed coded data transmission with relatively high immunity to error in the presence of different forms of transmission impairments.
A further object of the invention is a signal decoding method for providing an improved low error rate for Trellis and other convolutional encoders.
Other objects of the invention are a data transmission method and apparatus which are reliable at high data rates over telephone communications channels, which can be readily competitive with commercially available equipment, which require minimal additional hardware for its implementation, and which does not impair the functional basis for the encoding method with which it is employed.
Other objects of the invention will in part be obvious and will in part appear hereinafter.